Production of vehicle frames involves joining pipes together with bonding material. It is known to use resin for the bonding material (see JP-A -2001-278162, for example).
It is also known to employ welding for joining pipes or frame members together to produce vehicle frames (see JP-A-2002-173049, for example).
A joining method of JP-A-2001-278162 will be described with reference to FIG. 15.
In the method of joining together pipes shown in FIG. 15, a heat foamable adhesive S is applied to a pipe 101 constituting a frame member, and then the pipe 101 is press-fitted into a joint protrusion 103 of a lug 102. The joint protrusion 103 has ribs 103a, 103b on its inner surface. Thereafter, by heating the adhesive S by a heating means into a foam, the adhesive S extends into narrow gaps between the pipe 101 and the lug 102 at the joint, bonding the joint surfaces, which results in a simplified and economic frame member/pipe joining operation and an increased bonding strength.
In the above pipe joining method, however, the pipe 101 is press-fitted into the joint protrusion 103, which requires improving the precision of the outside diameter of the pipe 101 to which the heat foamable adhesive S is applied and the precision of the inside diameter of the ribs 103a, 103b of the joint protrusion 103, causing increased production costs.
Now, a joining method of JP-A-2002-173049 will be described with reference to FIG. 16.
A frame structure shown in FIG. 16 includes a first member 201, a second member 202, and an intermediate member 203 interposed between the first member 201 and the second member 202. These members are welded together. A foam filler 205 is put into a space 204 defined by the first member 201 and the intermediate member 203. The space 204 is filled with the foam filler 205 to provide a sufficient strength to the frame component.
The above frame component, however, has a structure in which the first member 201, second member 202 and intermediate member 204 are welded together, which requires that those members be made of the same material. Combination of different materials leads to reductions in weight and size, but combination of different materials takes time to weld, increasing production costs. Further, to contact members of different materials with one another at welds can cause galvanic corrosion at the contacts, making it difficult to maintain sufficient strength.
There is thus a demand for a frame joint structure and a joining method thereof which allow reduction in production costs and, increase in strength and prevention of galvanic corrosion.